Mn-Induced Thermal Stability of L1 Phase in Fept Magnetic Nanoscale Ribbons.

Magnetic nanoscale materials exhibiting the L1 tetragonal phase such as FePt or ternary alloys derived from FePt show most promising magnetic properties as a novel class of rare earth free permanent magnets with high operating temperature. A granular alloy derived from binary FePt with low Pt content and the addition of Mn with the nominal composition FeMnPt has been synthesized in the shape of melt-spun ribbons and subsequently annealed at 600 °C and 700 °C for promoting the formation of single phase, L1 tetragonal, hard magnetic phase. Proton-induced X-ray emission spectroscopy PIXE has been utilized for checking the compositional effect of Mn addition. Structural properties were analyzed using X-ray diffraction and diffractograms were analyzed using full profile Rietveld-type analysis with MAUD (Materials Analysis Using Diffraction) software. By using temperature-dependent synchrotron X-ray diffraction, the disorder-order phase transformation and the stability of the hard magnetic L1 phase were monitored over a large temperature range (50-800 °C). A large interval of structural stability of the L1 phase was observed and this stability was interpreted in terms of higher ordering of the L1 phase promoted by the Mn addition. It was moreover found that both crystal growth and unit cell expansion are inhibited, up to the highest temperature investigated (800 °C), proving thus that the Mn addition stabilizes the formed L1 structure further. Magnetic hysteresis loops confirmed structural data, revealing a strong coercive field for a sample wherein single phase, hard, magnetic tetragonal L1 exists. These findings open good perspectives for use as nanocomposite, rare earth free magnets, working in extreme operation conditions.